As energy costs rise, energy companies are increasingly concerned with optimizing their pumping unit systems, including reducing the amount of energy that a rod pumping system requires to operate and properly allocating costs between various wells. Referring to FIG. 1, in the prior art, rod pumping system 51 includes walking beam 52 pivotally supported by Samson post support assembly 68. Motor 53 connects to belt 61. Belt 61 connects to gearbox 54. Crank arm 60 connects to gearbox 54 and to pitman arm 69. Pitman arm 69 connects to walking beam 52. Walking beam 52 connects to horse head 55. Bridle 56 attaches to horse head 55 and to polished rod 57. Polished rod 57 connects to rod string 58 inside stuffing box 62. Rod string 58 connects to downhole pump 59. Electricity supply 63 connects to electricity meter 65 through transmission line 64. Electricity meter 65 connects to motor 53 through supply line 66.
A goal of the prior art has been to maximize the efficiency of rod pumping systems by reducing the amount of energy required to operate it. Such effects have included minimizing the real power required and energy losses from the power input of transmission line 64 to motor 53 to the useful power at downhole pump 59. A concurrent goal is to minimize the energy cost per barrel of oil produced in units of kilowatt hours per barrel (kWh/BBL). In furtherance of these goals the prior art has attempted to maximize the efficiency of motor 53, without complete success.
Alternating current (AC) power flow has the three components: active power, also known as true power (P), measured in watts (W); apparent power (S), measured in volt-amperes (VA): and reactive power (Q), measured in volt-amperes reactive (VAR).
The power factor is defined as:
                    F        =                              P            S                    .                                    Eq        .                                  ⁢        1            
In the case of a perfectly sinusoidal waveform, P, Q, and S can be expressed as vectors that form a vector triangle such that:S2=P2+Q2.  Eq. 2
If φ is the phase angle between current and voltage, then the power factor is equal to the cosine of the angle, cos φ, and:P=S cos φ.  Eq. 3
Since the units are consistent, the power factor is a dimensionless number between 0 and 1 for energy consumed and between −1 and 0 for energy generated. When the power factor is equal to 0, the energy flow is entirely reactive, and stored energy in the load returns to the source on each cycle. When the power factor is 1, all the energy supplied by the source is consumed by the load as real energy. Power factors are usually stated as “leading” or “lagging” to show the sign of the phase angle.
If a purely resistive load is connected to a power supply, current and voltage will change polarity in step, the power factor will be 1, and the electrical energy flows in a single direction across the network in each cycle. Inductive loads such as transformers and motors consume reactive power with current waveform lagging the voltage. Capacitive loads such as capacitor banks or buried cable generate reactive power with current phase leading the voltage. Both types of loads will absorb energy during part of the AC cycle, which is stored in the device's magnetic or electric field, only to return this energy back to the source during the rest of the cycle.
Electrical loads consuming alternating current power that are not purely resistive consume both real power and reactive power. The vector sum of real and reactive power is the apparent power. The presence of reactive power causes the real power to be less than the apparent power, and so, the electric load has a power factor of less than 1.
In the prior art, motor 53 is typically a three-phase AC induction motor running on 480 volts AC, 60 Hertz power. Motor 53 performs most efficiently and effectively during a heavily loaded, steady state operation. However, such an ideal operation of motor 53 is usually never attained because of the elasticity of rod string 58 and the geometry of rod pumping system 51. Due to the cyclic loading of rod pumping system 51, the instantaneous power requirements on motor 53 can be much higher or much lower than the average power required over one stroke of rod pumping system 51. When powering a perfectly mechanically balanced system—when the peak torque on gearbox 54 during the upstroke is equal to the peak torque on gearbox 54 during the downstroke—it is not uncommon for motor 53 to have periods during the stroke where the motor can experience states of overload, moderate load, no load, and negative load. Negative load means that rod pumping system 51 is driving motor 53 past its synchronous speed for a period of time. During such time, motor 53 acts as an asynchronous generator, putting power back into supply line 66.
Most oil and gas production companies experience negative loads, and consequently low power factors, caused by induction motors powering rod pumping systems. Electricity providers can penalize these companies for high demand and/or low power factors. It has been estimated that a lagging power factor, mostly caused by the induction motor, is responsible for as much as one-fifth of all grid losses in the United States, equivalent to 1.5% of total national power generation and costs on the order of $2 billion per year. Further, heating from high current flow causes transformers on both sides of electricity meters to fail. As a result, demand charges and power factor penalties are becoming more common for electric utility companies.
In the prior art, electricity meter 65 monitors the demand, energy usage, and power factor of rod pumping system 51 for well 67. The method typically used in the prior art to estimate power and energy consumption of rod pumping system 51 includes generating a random surface dynagraph card, typically by plotting load measurements of polished rod 57 versus position measurements of polished rod 57, calculating the average horsepower generated at polished rod 57, multiplying the horsepower by a loading factor and dividing the result by an estimated surface drive train efficiency, to obtain the input power of motor 53. However, the surface efficiency and the loading factor can change from stroke to stroke. Further, rod pumping system 51 does not produce identical surface dynagraph cards throughout the day. As a result, the method of the prior art is unreliable and inaccurate to measure and calculate the power and energy consumption, and power factor of for rod pumping system 51 for well 67. Further, this method of the prior art does measure or calculate the power and energy generated by the motor.
For multiple wells, rod pumping system 51 pumps at each well, all of which are connected to electricity meter 65. In this case, the power used to operate each rod pumping system 51, and how efficiently and effectively each rod pumping system 51 is using such power is of great importance. Further, if different wells have different owners or investors, then the allocation of energy consumption to each rod pumping system 51 is critical. Under prior art methods, energy consumption allocation is typically accomplished by totaling the energy consumed by each rod pumping system 51 connected to electricity meter 65 and dividing the total by the number of wells. This method is inaccurate because each rod pumping system 51 at each well differs in efficiency, power and energy consumed, power and energy generated, and power factor.
The prior art has attempted to address these problems, with limited success. For example, U.S. Pat. No. 5,204,595 to Opal er al. discloses controlling an electric drive motor coupled to reciprocating system by inserting a pair of power-off pulses in a reciprocation period of the motor energization cycle with one pulse in a top-of-cycle region and the other pulse in a bottom-of-cycle region to reduce rod stress and motor electrical power consumption and increase pump displacement and efficiency. However, the methods in Opal do not control the motor voltage by calculating a predicted RPM of the motor. Further, the methods do not correct a power factor of the motor or allocate energy consumption or energy generation among a plurality of rod pumping systems or determine electrical motor loading by calculating the RMS current on all three phases on a per stroke basis of the pumping unit and comparing the RMS current on all three phases to the full load current rating of the motor.
U.S. Pat. No. 5,284,422 to Turner et al. discloses monitoring and controlling a well pump apparatus with an electric motor. Electric current from the motor is measured periodically between the peak upstroke motor current and the peak downstroke motor current. Failure conditions are determined including sucker rod failure, counterweight loss or movement, and loss of a drive belt. The amount of work done by the pump well apparatus is determined. However, the determination of failure conditions in Turner is not useful in managing the energy consumption of the motor or the energy generation of the motor by controlling the motor voltage.
U.S. Pat. No. 5,661,386 Kueck er al. discloses a method and apparatus for assessing the efficiency of an in-service motor. The operating characteristics of the in-service motor are remotely measured and applied to an equivalent circuit to determine the performance characteristics of the in-service motor. The root mean square values of the voltage, current and their power factor are used to calculate rotor speed, power output, motor efficiency and torque of the electric induction motor. However, the Kueck method requires the use of an equivalent circuit that requires manual confirmation to evaluate the performance of the motor and thereby leads to a time consuming and costly method. Further, the method and apparatus in Kueck only detects and calculates deficient motor operating characteristics and provides no means for correcting such deficient motor operating characteristics.
U.S. Pat. No. 6,857,474 to Bramlett et al. discloses monitoring a reciprocating pump producing hydrocarbons from a well bore extending from the surface into the subterranean. The method includes generating a surface card and a downhole card. The method compares the generated surface card and the generated downhole card to “ideal” surface cards and downhole cards stored in a database to evaluate the energy consumption of the motor. However, the generated surface cards and downhole cards are inconsistent over time and thereby are prone to providing unreliable and inaccurate energy consumption data.
The prior art fails to disclose or suggest an apparatus and methods for accurately controlling a motor of a rod pumping system by measuring the current and voltage of the motor on a per stroke basis. Therefore, there is a need in the prior art for a system and methods for controlling a motor of a rod pumping system using previous RPMs of the motor and predicting an RPM of the motor; correcting a power factor of a motor of a rod pumping system; allocating energy consumption and allocating energy generation for a set of wells connected to an electricity meter using an amount of energy generated by each well; and generating an alert if a set of data of the motor is beyond a threshold for the set of data.